My invention relates to electronic illumination systems and, more particularly, to an improved solid-state inverter type ballast for starting and operating rapid-start type fluorescent lamps.
The fluorescent lamp is a known illumination device containing a phosphor coated glass tube confining an ionizable gas and a small amount of mercury, electron emitting cathodes, and having electrical terminals at each end, such that upon application of the proper electrical voltages to the terminals the gas becomes ionized and an electrical arc is established between the cathodes through the phosphor coated glass tube, evidenced by current flowing from the source, and the light radiation created by the arc energizes the phosphor coating which thereby fluoresces, generating diffused light. As is known, the fluorescent lamp is a peculiar type of an electrical load which possesses an electrical characteristic, termed by those in the art a "negative resistance". That is, a given high voltage is required to start the lamp and, once started, a lesser voltage is required to sustain its operation. By way of example, one conventional rapid-start type fluorescent lamp, the General Electric F40-T12 model, requires 280 volts peak for starting and for operation requires only 101 volts RMS and draws a current of 0.425 amps.
In present practice, and for some time, the apparatus used in the great majority of structures to provide these voltages to this kind of an electrical load is one which steps up the line voltages, such as 117 volts AC RMS, to the higher starting voltages and includes in a container a transformer of the high leakage reactance type containing a step-up secondary winding and a capacitive reactance in series to each lamp to provide the high starting voltages and, once the lamp starts, to limit current to "ballast" the lamp. Typically, each such lamp ballast contains the means to provide the operating voltage and current for at least two fluorescent lamps. The skilled reader recalls that there are at least two types of fluorescent lamps commonly found in existing lamp fixtures. One type is known as the "instant-start" lamp and the other type is known as the "rapid-start" type lamp. The former contains a special electron emitting cathode which emits electrons under the influence of the high voltage applied across the lamp terminals without the necessity of preheating the cathodes. However, in the latter type lamp the cathodes are of a less sophisticated construction; the lamp includes heaters or filaments through which electrical current is passed to generate heat and thereby warm up or heat the cathodes before the cathodes become sufficiently emissive. Prior practice with electromagnetic type ballast employed starter circuits providing the required short delay between the time heater current is supplied and the application of the high starting voltage across the lamp in order to ensure adequate lamp operating lifetime.
The present invention is particularly concerned with a solid-state ballast for fluorescent lamps of the rapid-start type and to the combination thereby formed. By solid-state ballast I refer to that class of devices which incorporate semi-conductor devices, such as transistors, and additional electrical components in circuits usually of the inverter-oscillator type, by means of which a low AC or DC voltage is transformed to the high-voltage levels required to operate one or more fluorescent lamps.
By way of example of these kinds of ballast, the prior art patent to Greene, U.S. Pat. No. 2,923,856, discloses a solid-state ballast incorporated within a container adapted to serve as a replacement directly for a comparable electromagnetic type transformer ballast. The Greene ballast includes an AC to DC rectifier for converting the line voltage, for example 117 volts AC RMS, to a lower level DC voltage normally specified for transistors, and transistors arranged in an oscillator circuit which in conjunction with a transformer is used to transform the AC currents developed in the transformer's primary by the action of the transistors up to the higher voltages which appear across the transformer's secondary to power the fluorescent lamps. Other types of solid-state ballast circuits appear in Schwartz, U.S. Pat. No. 3,005,130, and others. Specifically, there has been made known to me a particular inverter-oscillator circuit for use in supplying electrical loads in connection with a magnetron microwave energy generating source, as well as in connection with a fluorescent lamp which appears in the patent to Hester, U.S. Pat. No. 3,973,165, granted Aug. 3, 1976. Hester discloses an inverter-oscillator arrangement in which the 117 volt AC RMS is rectified by a bridge rectifier, smoothed by a filter circuit, and the derived DC voltage and current is applied through an inductor into the center tap of a transformer primary winding. Two transistors are arranged in a self-oscillatory circuit, including a parallel resonant circuit in the transformer's primary winding, to alternately conduct current in different directions through opposite halves of the transformer's primary winding for creating the alternating magnetic fields in the transformer's ferrite core, which, in turn, induces a high-voltage high-frequency voltage across a secondary winding for application across the electrical load, such as a capacitor in series with a fluorescent lamp of the instant-start type. An AC feed-back winding on the transformer is connected between the bases of the two transistors and as an additional low voltage secondary winding on the transformer provides AC which is rectified and filtered and applied as a DC bias to the bases of the transistors. The inherent current leakage associated with germanium-type transistors in Hester is used to start the circuit in oscillation as a known alternative to the inclusion of an additional resistor component connecting the transistor's bases directly to the input DC source via a high resistance path of the type found in other prior inverter-oscillator ballast devices. The Hester circuit includes ferrite transformer core containing a small air gap in the magnetic circuit to set the secondary inductance and thereby establish the desired operating frequency underload and assist to ensure a sinusoidal type secondary voltage waveform. Although the Hester circuit disclosed is particularly suited for use with instant-start type fluorescent lamps, those of ordinary skill recognize that the transformer may be modified without invention to include a plurality of conventional low-voltage secondary windings suited for connection to the heaters of rapid-start type fluorescent lamps in a conventional circuit. Ideally, rapid-start type lamps should not be started before the lamp's cathodes have been raised in temperature to a proper level recommended by the lamp manufacturer indirectly in terms of heater current stabilization. If a high enough starting voltage is applied across the lamp, however, simultaneously with the application of the heater voltage, electrical stresses are created on the lamp's cathodes which may commence to emit some electrons and start lamp operation before the cathodes have been fully raised to the temperature desired or as specified by the lamp manufacturer in terms of heater current stabilization, which stabilization I find typically requires a period of about at least 0.3 seconds over which the heater current drops from 3 amps to less than 1 amp for a F40 -T12 type rapid-start lamp, and this mode of operation appears to have been followed in connection with solid-state ballast circuits described in the patent literature. Although the previously described mode of operation does provide an illumination system, the effect is to reduce the operational lifetime of the rapid-start type lamp. Conceivably, a solution to reduced operating lifetime in such a mode is to provide lamps with more rugged cathode structures, which I understand to be available. To do so, however, requires a substantial price premium to be paid for the lamp.
Another consideration in fluorescent illumination systems is for the fluorescent lamps to provide a relatively constant light output during use. It is extremely annoying to users when the light output fluctuates noticeably and for no apparent reason. Those skilled in the art recognize that a cause of such fluctuation is fluctuation in the AC line voltage level, provided by the utility company, which supplies electrical power, indirectly, to the lamps. Solid-state ballasts which derive power from the AC lines are beset with those same variables.
A principal purpose of my invention therefore is to provide an improved solid-state ballast for use in connection with rapid-start type fluorescent lamps, which avoids application of a high starting voltage to the lamps until expiration of a desired cathode heating interval while providing cathode heating current. An ancillary object of my invention is to provide a solid-state AC ballast for starting and operating rapid-start type lamps that does not abruptly apply high starting voltage to the lamp so as to increase lamp operational life. A further purpose of the invention is to provide an AC solid-state ballast and lamp combination having some degree of regulation of lamp current as against line voltage variation. A more specific purpose is to improve upon the two transistor push-pull inverter-oscillator type ballast of the kind referred to in Hester patent U.S. Pat. No. 3,973,165 for providing better overall performance as part of a rapid-start type fluorescent lamp system.